Managing Swap Space

1. Viewing Swap Usage

Command	Description	Example Output
free -h	Displays memory and swap usage in a human-readable format	`total used free shared buff/cache available Mem: 7.7G 2.1G 3.2G 156M 2.4G 5.2G Swap: 2.0G 0B 2.0G`
swapon --show	Lists all active swap partitions and files with details	`NAME TYPE SIZE USED PRIO /swapfile file 2.0G 0B -2`
cat /proc/swaps	Displays detailed information about active swap spaces	`Filename Type Size Used Priority /swapfile file 2097152 0 -2`
vmstat	Shows virtual memory statistics including swap usage	`procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu----- r b swpd free buff cache si so bi bo in cs us sy id wa st 0 0 0 3298560 123456 456789 0 0 12 34 56 78 9 1 90 0 0`

Example: Check current swap usage with free

free -h

Example: List all active swap spaces

swapon --show

2. Creating Swap Partition

Step	Description	Action
1	Check current swap usage	`swapon --show`
2	Identify a suitable disk partition or file	`lsblk`
3	Create a swap file (e.g., 1GB)	`dd if=/dev/zero of=/swapfile bs=1M count=1024`
4	Set appropriate permissions	`chmod 600 /swapfile`
5	Set up the swap space	`mkswap /swapfile`
6	Enable the swap file	`swapon /swapfile`
7	Make swap persistent across reboots	Add to `/etc/fstab`: `/swapfile none swap sw 0 0`

Example: Enabling Swap

swapon /swapfile

Note: Ensure the swap file is not accessible to other users by setting strict permissions.

3. Enabling Swap Partition

Command	Description	Notes
sudo swapon /dev/sdX2	Enable a swap partition	Replace sdX2 with actual device name
sudo swapon --show	Display active swap space	Verify swap is enabled
sudo swapon --all	Enable all swap partitions listed in /etc/fstab	Useful for system boot

Example: Enabling Swap Partition

# List current swap partitions
sudo swapon --show

# Enable a specific swap partition
sudo swapon /dev/sdb2

# Verify it's active
sudo swapon --show

Note: Ensure the swap partition is properly formatted and added to /etc/fstab for persistence across reboots.

4. Creating Swap File

Example: Create a 1GB Swap File

# Create a 1GB swap file
sudo fallocate -l 1G /swapfile

# Set proper permissions
sudo chmod 600 /swapfile

# Set up the swap space
sudo mkswap /swapfile

# Enable the swap file
sudo swapon /swapfile

Command	Description	Notes
fallocate -l 1G /swapfile	Creates a 1GB file with zeroed contents	Alternative: `dd if=/dev/zero of=/swapfile bs=1M count=1024`
chmod 600 /swapfile	Sets file permissions to owner-only read/write	Required for security; swap files must not be readable by others
mkswap /swapfile	Set up the file as a swap space	Must be run before enabling it with swapon
swapon /swapfile	Enable the swap file immediately	Changes are lost on reboot unless added to /etc/fstab

Note: The swap file must not be readable by other users for security reasons.

Warning: Using fallocate is preferred over dd because it allocates space more efficiently and avoids filling the disk with zeros.

Example: Make Swap File Permanent

# Add to /etc/fstab for persistence across reboots
echo '/swapfile none swap sw 0 0' | sudo tee -a /etc/fstab

# Verify swap is active
sudo swapon --show

5. Setting Up Swap File

Creating a swap file is a common way to add virtual memory to a Linux system. Below are the steps and configurations required to set up a swap file.

Step	Description	Action
1. Check Current Swap	Verify existing swap usage	`swapon --show`
2. Create Swap File	Allocate space for swap (e.g., 1GB)	`sudo fallocate -l 1G /swapfile`
3. Set Permissions	Secure the swap file to prevent unauthorized access	`sudo chmod 600 /swapfile`
4. Format as Swap	Initialize the file for use as swap space	`sudo mkswap /swapfile`
5. Enable Swap	Activate the swap file	`sudo swapon /swapfile`
6. Make Permanent	Ensure swap is enabled after reboot	`echo '/swapfile none swap sw 0 0' \| sudo tee -a /etc/fstab`

Example: Verify Swap Activation

free -h

Output should show the new swap space in the "Swap" row.

Note: It is recommended to set the swap file permissions to 600 to prevent unauthorized access.

Warning: Creating a large swap file can consume significant disk space. Ensure sufficient storage is available.

Swap File Configuration Summary

Size: Typically 1-2x RAM size for systems with less than 4GB RAM
Location: Preferably on a fast SSD or separate partition
Permissions: Must be 600 to prevent access by non-root users
Activation: Use swapon to activate and fallocate to create

6. Configuring Permanent Swap

Step	Description	Action
1	Check current swap status	`swapon --show`
2	Create a swap file (e.g., 1GB)	`sudo fallocate -l 1G /swapfile`
3	Set proper permissions	`sudo chmod 600 /swapfile`
4	Set up the swap space	`sudo mkswap /swapfile`
5	Enable the swap file	`sudo swapon /swapfile`
6	Make it permanent by editing /etc/fstab	`echo '/swapfile none swap sw 0 0' \| sudo tee -a /etc/fstab`

Note: Ensure the swap file is not readable by other users for security reasons.

7. Disabling Swap

Command	Description	Notes
sudo swapoff -a	Disables all swap spaces currently in use	Does not remove swap space from /etc/fstab
sudo swapoff /dev/sdXn	Disables a specific swap partition	Replace sdXn with actual device name

Example: Disable all swap spaces

sudo swapoff -a

Note: Disabling swap does not remove it from /etc/fstab. To permanently disable, edit /etc/fstab to comment out or remove the swap entry.

8. Adjusting Swap Priority

Parameter	Description	Default Value	Notes
/proc/sys/vm/swappiness	Controls the tendency of the kernel to swap pages	`60`	Range: 0-100. Lower values mean less swapping
/proc/sys/vm/swappiness	Value of 0 means never swap	`0`	Not recommended for most systems
/proc/sys/vm/swappiness	Value of 100 means always swap	`100`	Can cause performance issues

Example: Setting swappiness to 10

sudo sysctl vm.swappiness=10

Example: Making swappiness persistent across reboots

echo "vm.swappiness=10" | sudo tee -a /etc/sysctl.conf

Note: A swappiness value of 10 is generally recommended for desktop systems to avoid excessive swapping while still providing good memory management.

9. Tuning Swappiness

Parameter	Value Range	Description	Default
vm.swappiness	`0` - `100`	Controls the preference of the kernel to swap out pages vs. reclaiming memory	`60`

Example: Setting Swappiness to 10

sudo sysctl vm.swappiness=10

Example: Making Swappiness Change Permanent

echo "vm.swappiness = 10" | sudo tee -a /etc/sysctl.conf

Note: Lower values (e.g., 10) mean the system will try to avoid swapping pages, preferring to reclaim memory from caches and buffers instead.

Warning: Setting vm.swappiness to 0 can lead to performance issues under memory pressure as the system might not swap out less-used pages effectively.

10. Monitoring Swap Activity

Command	Description	Notes
free -h	Displays memory usage including swap	Shows total, used, and available memory in human-readable format
vmstat	Reports virtual memory statistics	Includes swap in/out rates and memory usage details
sar -S	System activity reporter for swap	Provides historical data on swap usage over time
iostat -x	I/O statistics including swap activity	Shows detailed I/O metrics related to swap operations

Example: Check current swap usage

free -h
              total        used        free      shared  buff/cache   available
Mem:           7.7G        2.1G        3.2G        156M        2.4G        5.2G
Swap:          2.0G          0B        2.0G

Example: Monitor swap usage over time

vmstat 1 5
procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 0  0      0 3285644 123456 2456789    0    0   123   456   123   456  5  3 92  0  0
 0  0      0 3285644 123456 2456789    0    0   123   456   123   456  5  3 92  0  0
 0  0      0 3285644 123456 2456789    0    0   123   456   123   456  5  3 92  0  0
 0  0      0 3285644 123456 2456789    0    0   123   456   123   456  5  3 92  0  0
 0  0      0 3285644 123456 2456789    0    0   123   456   123   456  5  3 92  0  0

Note: The si and so columns in vmstat show swap in/out rates. If these values are consistently high, it indicates frequent swapping which can degrade performance.